Abstract

Recently, mode instability was observed in optical fiber lasers at high powers, severely limiting power scaling for single-mode outputs. Some progress has been made towards understanding the underlying physics. A thorough understanding of the effect is critical for continued progress of this very important technology area. Mode instability in optical fibers is, in fact, a manifestation of stimulated thermal Rayleigh scattering. In this work, a quasi-closed-form solution for the nonlinear coupling coefficient is found for stimulated thermal Rayleigh scattering in optical fibers. The results help to significantly improve understanding of mode instability.

Nonlinear coupling coefficient χ at the peak of real part of χ and the corresponding frequency fmax for a step index fiber with 2b = 400μm, 2a = 2d = 30μm, dependence on (a) V (NA is varied to change V) and (b) fraction of the doped radius d/a at NA = 0.06.

Nonlinear coupling coefficient χ at the peak of real part of χ and the corresponding frequency fmax for step index fibers with 2b = 400μm and 2a = 2d, core diameter is varied while V is kept constant for each lines (NA is varied to keep V constant).